As is well known in the field of frequency control, crystal resonators are used for providing highly precise frequency references for many applications. Piezoelectric quartz crystal resonators are commonly implemented in oscillators to provide highly accurate timing signals for communications, navigation and radar applications.
However, it is also well known that crystal resonators are susceptible to acceleration forces. For example, if a resonator is placed in a vibrating environment, the frequency of the resonator will be perturbed as a function of the vibration level. This degrades the stability of the resonator, and can compromise the performance of the system in which it is used. The poor performance is the result of frequency shifts and timing errors that occur when the resonator is subjected to stresses caused by acceleration or gravity.
The acceleration sensitivity of a resonator arises from forces imparted on the resonator element from the surrounding enclosure. These forces are transferred to the resonator through the mount structure. Rigidity and asymmetry of the mounting clips, manufacturing imperfections, and acoustic mode offset can result in a misalignment of the acceleration stress field and the acoustic mode center which has been shown to adversely impact acceleration sensitivity and degrade performance.
In a typical industry package, there is a package floor with posts extending to the outside. Flexible metal clips are connected from the post to the crystal and they retain the crystal. The clips are flexible and can be bent in a variety of ways. The clips are typically metal and conductive, and the quartz crystal is affixed to the clips. In conventional mounting configurations, the header is typically made of glass, together with a material such as Kovar, which is thermal expansion-coefficient matched to the sealing glass to provide a hermetic enclosure. The clips that connect the posts and the resonator can be made of a variety of materials, such as nickel or stainless steel. In either case, the thermal expansion coefficient of the quartz disc will not be matched to that of the header assembly. Crystal manufacturing processes usually involve high temperature operations such as adhesive curing, so as the assembly cools, the thermal expansion-coefficient mismatch produces residual stress applied to the resonator disc. And, this stress bias can result in an asymmetry or misalignment of the acceleration stress field and acoustic mode center.
There are existing systems that allow sufficient flexibility and retain the crystal by longer clips. However, these clips prohibit a low profile and are therefore inappropriate for certain applications requiring lower profiles. In addition, mechanical stresses on the clips are transmitted directly to the resonator. In prior mounting systems, attempts have been made to lower the packaging profile by making the clips shorter and shorter. But, the shortened clips are very stiff and do not adequately provide sufficient flexibility, and performance is degraded.
In one state-of-the-art embodiment, the crystal blank is mounted with its face perpendicular to the package terminations, and long mounting clips to attach the resonator plate. The mounting clips are in the plane of the crystal but are not symmetrical. Typically only two clips are utilized. Examples of this form of package header, whose bases are typically oval in shape, are the cold-welded types HC47/U, HC43/U and HC45/U. In such mount designs, the resonator is held perpendicular to the package header and retained by the longer clips, which reduces the stress, but increases the package profile. There are other prior configurations in which the crystal clips contact the blank at an angle ranging from around 40 degrees to substantially perpendicular or 90 degrees. In this type of configuration there are generally four clips used, but the mounting forces are not in the plane of the resonator. The resonator is held parallel to the package header and the clips keep the resonator in place, and the crystal face is parallel to the terminations. Examples of this form of TO-X holder, which are typically circular, are the cold-welded types HC40/U, HC37/U and HC35/U. Because of the possibility of four mounting points, and the short length of the mounts, the circular styles are particularly suited to low profile applications and to situations where the environmental conditions are harsh.
In a symmetrical mount, the stress field is symmetric with respect to the acoustic mode. In theory, if the stress field center from the symmetrical mount and the acoustic mode center are coincident, the sensitivity to vibration should be minimized. However, packaging imperfections and manufacturing stresses can create a misalignment of the stress field and acoustic mode center. For example, epoxy problems or shrinkage on a single clip may asymmetrically induce stress and cause misalignment. Mis-positioning of the resonator blank can also cause misalignment of the stress field and acoustic mode center.
The in-plane acceleration sensitivity of a plano-plano or bi-convex resonator, regardless of orientation, will vanish to the first order for a mounting structure that is perfectly symmetric with respect to the center of the acoustic mode shape. And for a plano-convex resonator, the in-plane acceleration sensitivity will nearly vanish, for example, a few parts in 1012 per g. This is supported by the articles by Tiersten and Zhou, entitled “An Analysis of the In-Plane Acceleration Sensitivity of Contoured Quartz Resonators with Rectangular Supports”, Proceedings of the 44th IEEE International Frequency Control Symposium, pp 461–467, 1990; and “The Increase in the In-Plane Acceleration Sensitivity of the Plano-Convex Resonator Due to Its Thickness Asymmetry”, Proceedings of the 45th IEEE International Frequency Control Symposium, pp. 289–297, 1991.
Also explained by Zhou and Tiersten, the normal acceleration vanishes to the first order for a perfectly symmetric structure, see “On the Influence of a Fabrication Imperfection on the Normal Acceleration Sensitivity of Contoured Quartz Resonators with Rectangular Supports”, Proceedings of the 44th IEEE International Frequency Control Symposium, pp. 452–460, 1990. In fact, the normal acceleration sensitivity will increase linearly with offset. Thus alignment of the centers is extremely important in achieving low g-sensitivity.
In practice, however, this goal has been difficult to achieve, particularly in the context of the industry standard package styles readily available for mounting precision crystals.
Referring to prior art FIG. 1, FIG. 2, and FIG. 3, these figures show prior configurations that have two, three, and four point mounting locations, respectively. The resonator 5 is supported with thin metal clips 10, or ribbons, which extend down to the package base 25. In these conventional mounting systems, the mount is designed to accommodate differing thermal expansion coefficients between the package 25, the crystal material 5 and the mount clips 10. This is achieved by making the distance between the resonator 5 and the package base 25 sufficiently long or by making the material sufficiently compliant. This provides a stress relief effect, which is necessary for obtaining stability criteria such as good frequency aging performance. One of the drawbacks of this technique is that symmetry of the mounting structure is compromised, and under acceleration, the long and compliant clips will deform leading to asymmetry of the mounting structure. This, in turn, will lead to a misalignment of the acoustic mode and symmetric support centers. For the case of the three-point configuration (FIGS. 2a, b), the symmetry is further compromised to simplify assembly.
Manufacturing tolerances when assembling these types of resonators can also lead to a wide variation in acceleration sensitivity. Misalignment of the resonator mode and support centers, tilt of the blank with respect to the crystal base, and variations in the amounts of adhesive can all lead to a wide range of acceleration sensitivity for a given batch of resonators. Alignment of the blank is normally a manual process and the adhesive application can vary considerably from resonator to resonator. This can all lead to offset of the resonator mode and mount centers.
The importance of alignment of the mount and acoustic mode center is further stressed by the work done by EerNisse et al. which is described in U.S. Pat. No. 5,168,191; U.S. Pat. No. 5,022,130; U.S. Pat. No. 4,935,658; and U.S. Pat. No. 4,837,475, as well as in the following articles: E. P. EerNisse, L. D. Clayton, and M. H. Watts, “Distortions of Thickness Shear Mode Shapes in Plano-Convex Quartz Resonators with Mass Perturbations,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 37, no. 6, pp. 571–576, Nov. 1990; E. P. EerNisse, R. W. Ward, M. H. Watts, R. B. Wiggins, O. L. Wood, “Experimental Evidence for Mode Shape Influence on Acceleration-Induced Frequency Shifts in Quartz Resonators,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 37, no. 6, pp. 566–570, November 1990.
As disclosed in these various EerNisse articles and patents, the subject matter pertained to a process to move the acoustic mode center on finished resonator assemblies to improve acceleration sensitivity. Based on testing, there was an observed frequency shift caused by a 150 Angstrom film of Platinum deposited in a small pie-shaped area as a probe for mapping out the acoustic mode shape. Once the mode shape was mapped out, a patch of platinum was deposited onto the quadrant with the least amount of acoustic strength. The added mass resulted in greater energy trapping causing the mode center to move in the direction of the added mass. This resulted in a redistribution of the acoustic mode and a corresponding improvement in the acceleration sensitivity.
Although the technique results in improved acceleration sensitivity (low parts in 10^10 per g of acceleration), the disadvantages of this technique are manufacturing time and cost. Probing a resonator with small patches of platinum and then depositing onto the weakest quadrant requires special tooling and considerable equipment time. Much time is also needed for quantifying the acceleration sensitivity numerous times.
Another effect that has not been explored in detail is the effect of residual static stresses on the dynamic acceleration sensitivity. The majority of the existing theory assumes that the resonator has a zero dc stress bias when the analysis is performed. In reality, residual static stresses may reside in the resonator due to the mounting clips and the adhesive expanding and contracting throughout the manufacturing process.
One approach to reducing these biasing stresses, as well as reducing the coupling of vibratory stresses, would be to use a compliant mounting structure, as proposed in the article by R. D. Weglein, “The Vibration-Induced Phase Noise of a Visco-Elastically Supported Crystal Resonator”, Proceedings of the 43rd IEEE International Frequency Control Symposium, pp. 433–438, 1989. Weglein showed that low values of acceleration sensitivity could be achieved by using a visco-elastic adhesive for attaching the crystal to four rectangular mounting posts. Total Gamma values (<=) 3×10−10 per g were reproducibly achieved for 100 MHz, 5th overtone resonators. The advantages of this technique are two-fold. First, any residual stresses due to manufacturing will relax in the compliant adhesive. Secondly, vibration that is normally coupled into the resonator through the mounting structure would be greatly reduced. However, the aging characteristics of the resonator may suffer due to the out-gassing properties of most compliant adhesives.
Other works have demonstrated the importance of how stress can be coupled into a resonator causing a corresponding shift in the resonant frequency. In-plane diametric forces applied to the edge of a crystal resonator produce frequency shifts that are dependent upon the azimuthal angle Ψ in the plane of the plate. This effect has been called the force-frequency effect and is thoroughly described in the article by A. Ballato, E. P. EerNisse, and T. Lukaszek, “The Force-Frequency Effect in Doubly Rotated Quartz Resonators”, Proceedings of the 31st IEEE International Frequency Control Symposium, pp. 8–16, 1977. Their experimental and theoretical works demonstrated that the location of the mounting clips on the edge of the crystal resonator with respect to the resonators crystallographic x axis could be optimized to minimize the force-frequency effect. The optimal Ψ angle and optimal clip arrangement was found to be dependent on the cut of quartz.
This clearly can be utilized to reduce the sensitivity of a resonator to an acceleration field, but does not account for gross misalignment or significant amounts of pre-biasing stresses that may exist within the resonator's support structure.
There have been many attempts to alleviate the aforementioned problems. In U.S. Pat. No. 4,406,966 there is a temperature compensated system that uses a spring or bellows support to connect to the resonator. The flexible bellows or springs have a coefficient of thermal expansion that is different than that of the resonator. The spring or bellows acts as a shock absorber to mitigate the vibration of the resonator. This system requires adding additional components to the package adding complexity and cost.
The resonator mounting of U.S. Pat. No. 4,639,632 describes a pair of lead-in conductors retaining a resonator in a flat package. The lead-in conductors contact one surface of the resonator and provide electrical connections to the outside of the package. There are U shaped portions shown that are intended to extend the heat conducting path from the external portions of the lead-in conductor to the projections contacting the crystal.
In U.S. Pat. No. 3,828,210, there is a mounting structure designed for housing one or more crystal plates. The housing has upper mount tabs that are ‘L’ shaped to provide some resiliency for thermal expansion as the connection is at the leg of the tab.
The temperature insensitive mounting described in U.S. Pat. No. 4,430,596 discloses using pedestals located at sweet spots, or axes in the X-Z plane of the crystal that are less sensitive to stresses. The axes at 60 degrees, 120 degrees and 240 degrees and 300 degrees were found to be insensitive to stresses generated in the crystal by thermal expansion of the substrate and crystal.
Stemming from the work reported by A. Ballato, E. P. EerNisse, and T. Lukaszek, “The Force-Frequency Effect in Doubly Rotated Quartz Resonators”, Proceedings of the 31st IEEE International Frequency Control Symposium, pp. 8–16, 1977, several new resonator structures were designed to take advantage of the optimal mounting angles Ψ for various quartz resonator types. This is thoroughly described by T. Lukaszek and A. Ballato, “Resonators for Severe Environments”, Proceedings of the 33rd IEEE International Frequency Control Symposium, pp. 311–321, 1979 and in U.S. Pat. No. 4,454,443.
The quartz resonators were physically cut such that the optimal mounting angles would be achieved with a much wider mounting surface thereby reducing the concentration of stress caused by having a small mount point. The drawback of this technique is that the aligrnent and cutting of the resonators further complicates the manufacturing process increasing the process time and resonator cost.
Despite all the previous attempts in the art, there continues to be a need for improvement in the packaging of resonator elements to achieve improved symmetry and stress compensation resulting in improved aging, pressure sensitivity, and acceleration sensitivity.